1. Field of the Invention
The present invention relates to a thin film transistor having an organic semiconductor film and a manufacturing method thereof. In addition, the present invention relates to a semiconductor device provided with the above thin film transistor and a manufacturing method thereof.
2. Description of the Related Art
A display device provided with thin film transistors (TFTs) has been studied recent years. The display device provided with TFTs can be operated in low-power consumption and occupy small space compared with CRT, therefore it has been used for a display portion of a personal computer, a PDA, or the like. These TFTs have been practically manufactured using inorganic semiconductor materials such as amorphous silicon, crystalline silicon, or the like. However, there is a problem that lots of effective substrate materials can't be used since its processing temperature is above 350° C. when TFTs are manufactured using inorganic semiconductor materials.
Consequently, in addition to the inorganic materials, a thin film transistor having an organic semiconductor film formed by organic materials (hereafter referred to as an organic TFT) has been researched. Since an organic TFT can be formed at low temperature, plastic materials can be used for the substrate. As a result, a light and flexible device can be obtained. Furthermore, the organic TFT has advantages of low production cost for the sake of being formed on the inexpensive substrate materials and low voltage for driving the device.
Dipping, casting, bar coating, spin coating, spraying, ink jetting or printing is applied for the organic semiconductor film which use a polymer type (high-molecular type) organic material. (For example, Japanese Patent Laid Open No. 2000-29403, No. 2000-269504). Also, vapor deposition or the like is applied for the organic semiconductor film which use a low-molecular type organic material. (For example, Japanese Patent Laid Open No. 8-228035, No. 10-125924). In terms of improving uniformity in a film thickness, spin coating and vapor deposition are often used.
However, there have been problems that any of the above manufacturing methods requires a special apparatus, and that formed organic semiconductor films are so little to quantity consumed materials and most of the used materials are discarded in the end. Moreover, maintenance of apparatus such as cleaning of the inside of an apparatus cup or a chamber has needed to be continually performed in order to remove the contamination resulting from the material that is wastefully discarded. Therefore, a great cost for materials and man-hours for maintenance of apparatus have been required. As a result, it has not been desirable to apply these methods not only for the effect on the production cost, but also the disastrous effect on the environment generated from material wastes and liquid wastes.
It is difficult to coat the films separately by spin coating, so that patterning is required after coating the organic films allover. It is still difficult to perform patterning with absolute precision by this method. Also, there is a possibility of a thermal decomposition since temperature of sublimation is close to that of the thermal decomposition in some organic materials. In addition, ink jetting, printing and some other ways have not yet come into practical use.
Moreover, in the methods above, it is difficult to form a thin and uniform organic semiconductor film and to attain a TFT characteristic.